Various electronic apparatuses, such as mobile telephones and car navigation systems, have been recently improved in functional performance and versatility. Such electronic apparatuses are accordingly equipped with light-transmittable touch panels mounted on display screens of display devices, such as liquid crystal displays. Users activate the touch panels with fingers or pens while viewing the screens of the display devices through the touch panels for switching functions of the electronic apparatuses. The touch panels have been accordingly demanded to be activated easily while allowing the display devices to be clearly viewed through the touch panels.
FIG. 8 is a cross sectional view of conventional touch panel 501 disclosed in Japanese Patent Laid-Open Publication No. 2006-107015. Upper substrate 1 is made of film of light-transmittable, insulating resin, such as polycarbonate. Lower substrate 2 is made of film of light-transmittable, insulating resin. Upper resistor layer 3 made of light-transmittable, resistor material, such as indium tin oxide, is provided on a lower surface of the upper substrate 1. A lower resistor layer 4 made of light-transmittable, resistor material, such as indium tin oxide, is provided on an upper surface of the lower substrate 2. Dot spacers 12 made of insulating resin are provided on an upper surface of the lower resistor layer 4 by equal intervals. A pair of upper electrodes are provided at both ends of the upper resistor layer 3, respectively, and arranged along a predetermined direction. A pair of lower electrodes are provided at both ends of the lower resistor layer 4, respectively, and arranged along a direction perpendicular to the predetermined direction along which the upper electrodes are arranged. Spacer 5 having substantially a frame shape is coated with adhesives on upper and lower surfaces or one of the surfaces. The adhesives adhere upper substrate 1 to lower substrate 2 via spacer 5, thus allowing the upper resistor layer 3 and the lower resistor layer 4 to face each other by a distance.
Polarizing plate 6 includes a polyvinyl alcohol sheet which contains iodine or dye and which is orientated, and tri-acetyl cellulose layers provided on upper and lower surfaces of the sheet. Polarizing plate 6 is bonded with adhesive layer 7 to the upper surface of upper substrate 1, thus providing touch panel 501.
Touch panel 501 is mounted onto a display screen of a display device, such as a liquid crystal display, and has the upper and lower electrodes connected to an electronic circuit of an electronic apparatus.
A user presses polarizing plate 6 with a finger of his/hers or a pen while viewing the display screen of the display device through touch panel 501. Upper substrate 1 accordingly sags together with polarizing plate 6, and causes a portion of upper resistor layer 3 under the pressed portion to contact lower resistor layer 4.
The electronic circuit applies a voltage between the upper electrodes, and detects a potential at the potion of the upper resistor layer 3 contacting the lower resistor layer 4 through lower resistor layer 4 and the lower electrodes. The electronic circuit further applies a voltage between the lower electrodes, and detects a potential at the portion of the upper resistor layer 3 contacting lower resistor layer 4 through upper resistor layer 3 and the upper electrodes. The electronic circuit then determines the position of the pressed portion based on the detected potentials. The electronic apparatus switches functions according to the determined position.
Upon receiving external light, such as sunlight or illumination, on an upper surface of polarized plate 6, polarizing plate 6 converts the external light into light polarized along either an X-direction or a Y-direction perpendicular to the X-direction. The polarized light has intensity substantially half of that of the external light. The polarized light is transmitted from the polarizing plate 6 to the upper substrate 1. The polarized light passes through upper substrate 1, and then reflected by lower resistor layer 4. Since the reflected light has the intensity reduced substantially to the half with polarizing plate 6, light emitted from the upper surface of the polarizing plate 6 has small intensity. This allows the user to view the display screen of the display device without interference of the external light.
When conventional touch panel 501 is used under a hostile environment at high temperatures, e.g., in an automobile under direct sunlight, adhesive layer 7 may have air bubbles 8 produced inside. Air bubbles 8 reflect the external light, and accordingly prevent the user from viewing the display device clearly.